Atomic force microscope images of lipid layers spread from vesicle suspensions

Structure and distribution of the Bacillus thuringiensis Cry4Ba toxin in lipid membranes

Bacillus thuringiensis Cry delta-endotoxins cause death of susceptible insect larvae by forming lytic pores in the midgut epithelial cell membranes. The 65 kDa trypsin activated Cry4Ba toxin was previously shown to be capable of permeabilizing liposomes and forming ionic channels in receptor-free planar lipid bilayers. Here, magnetic ACmode (MACmode) atomic force microscopy (AFM) was used to characterize the lateral distribution and the native molecular structure of the Cry4Ba toxin in the membrane. Liposome fusion and the Langmuir-Blodgett technique were employed for supported lipid bilayer preparations. The toxin preferentially inserted in a self-assembled structure, rather than as a single monomeric molecule. In addition, the spontaneous insertion into receptor-free lipid bilayers lead to formation of characteristic pore-like structures with four-fold symmetry, suggesting that tetramers are the preferred oligomerization state of this toxin.

Preferential orientation of an immunoglobulin in a glycolipid monolayer controlled by the disintegration kinetics of proteo-lipidic vesicles spread at an air–buffer interface

The insertion of immunoglobulin (IgG) in a glycolipid monolayer was achieved by using the ability of new proteo-glycolipid vesicles to disintegrate into a mixed IgG-glycolipid interfacial film after spreading at an air-buffer interface. The interfacial disintegration kinetics was shown to be directly dependent on the initial vesicle surface density and on the buffer ionic strength. The presence of the immunoglobulin in the glycolipid film was displayed by an increase of the lateral compressibility (Cs) during monolayer compression. Cs magnitude modifications, due to the antibody effect on the monolayer packing, decreases as the spread vesicle density increases. At interfacial saturation, the lateral compressibility profile becomes similar to that of a control monolayer without antibody. However, the careful analysis of the mixed monolayer after transfer by Langmuir-Blodgett technique (ATR-FTIR characterisation, enzyme immunoassociation) clearly demonstrated that the antibody was still present in such conditions and was not completely squeezed out from the interface as compressibility changes could have meant. At nonsaturating vesicle surface density, IgG molecules initially lying in the lipid matrix with the Y-shape plane parallel to the interface move to a standing-up position during the compression, leading to lateral compressibility modifications. For a saturating vesicle surface density, the glycolipid molecules force the IgG molecules to directly adopt a more vertical position in the interfacial film and, consequently, no lateral compressibility modification was recorded during the compression.

Growth of cationic lipid toward bilayer lipid membrane by solution spreading: scanning probe microscopy study

The growth of cationic lipid dioctadecyldimethylammonium bromide (DODAB) toward bilayer lipid membrane (BLM) by solution spreading on cleaved mica surface was studied by atomic force microscopy (AFM). Bilayer of DODAB was formed by exposing mica to a solution of DODAB in chloroform and subsequently immersing into potassium chloride solution for film developing. AFM studies showed that at the initial stage of the growth, the adsorbed molecules exhibited the small fractal-like aggregates. These aggregates grew up and expanded laterally into larger patches with time and experienced from monolayer to bilayer, finally a close-packed bilayer film (5.4+/-0.2 nm) was approached. AFM results of the film growth process indicated a growth mechanism of nucleation, growth and coalescence of dense submonolayer, it revealed the direct information about the film morphology and confirmed that solution spreading was an effective technique to prepare a cationic bilayer in a short time.

The ring structure and organization of light harvesting 2 complexes in a reconstituted lipid bilayer, resolved by atomic force microscopy

The main function of the transmembrane light-harvesting complexes in photosynthetic organisms is the absorption of a light quantum and its subsequent rapid transfer to a reaction center where a charge separation occurs. A combination of freeze-thaw and dialysis methods were used to reconstitute the detergent-solubilized Light Harvesting 2 complex (LH2) of the purple bacterium Rhodopseudomonas acidophila strain 10050 into preformed egg phosphatidylcholine liposomes, without the need for extra chemical agents. The LH2-containing liposomes opened up to a flat bilayer, which were imaged with tapping and contact mode atomic force microscopy under ambient and physiological conditions, respectively. The LH2 complexes were packed in quasicrystalline domains. The endoplasmic and periplasmic sides of the LH2 complexes could be distinguished by the difference in height of the protrusions from the lipid bilayer. The results indicate that the complexes entered in intact liposomes. In addition, it was observed that the most hydrophilic side, the periplasmic, enters first in the membrane. In contact mode the molecular structure of the periplasmic side of the transmembrane pigment-protein complex was observed. Using Föster's theory for describing the distance dependent energy transfer, we estimate the dipole strength for energy transfer between two neighboring LH2s, based on the architecture of the imaged unit cell.

Advances in the characterization of supported lipid films with the atomic force microscope

During the past decade, the atomic force microscope (AFM) has become a key technique in biochemistry and biophysics to characterize supported lipid films, as testified by the continuous growth in the number of papers published in the field. The unique capabilities of AFM are: (i) capacity to probe, in real time and in aqueous environment, the surface structure of lipid films; (ii) ability to directly measure physical properties at high spatial resolution; (iii) possibility to modify the film structure and biophysical processes in a controlled way. Such experiments, published up to June 2000, are the focus of the present review. First, we provide a general introduction on the preparation and characterization of supported lipid films as well as on the principles of AFM. The section 'Structural properties' focuses on the various applications of AFM for characterizing the structure of supported lipid films: visualization of molecular structure, formation of structural defects, effect of external agents, formation of supported films, organization of phase-separated films (coexistence region, mixed films) and, finally, the use of supported lipid bilayers for anchoring biomolecules such as DNA, enzymes and crystalline protein arrays. The section 'Physical properties' introduces the principles of force measurements by AFM, interpretation of these measurements and their recent application to supported lipid films and related structures. Finally, we highlight the major achievements brought by the technique and some of the current limitations.

Interaction of phospholipid vesicles with smooth metal-oxide surfaces

The interaction of phospholipid vesicles with planar metal oxide supports has been previously reported as a means of preparing supported lipid bilayers, which are useful models of biological membranes. Nevertheless, extant evidence that bilayers are actually formed is rather circumstantial, and the necessary and sufficient conditions for their formation have never been delineated. Here, we tackle this problem by using smooth planar optical waveguides as the support. Analysis of the lightmode spectra of the waveguides, measured in situ during the deposition process, yields the mass of lipid deposited at the solid/liquid interface. By comparing the optogeometric parameters of the structures assembled from the vesicles with those of a lipid bilayer of known structure assembled using the Langmuir-Blodgett technique, we show that in many cases the vesicles remain intact and form a supported layer of vesicles rather than a bilayer, and often mixed structures (intact vesicles embedded in a bilayer partially covering the surface) occur. Careful analysis of the lipid deposition kinetics corroborates this result. We have also found that divalent cations dramatically promote attachment of mixed phosphatidylcholine/phosphatidylglycerol vesicles to form supported vesicle layers, and bilayer formation from pure phosphatidylcholine vesicles.

The height of biomolecules measured with the atomic force microscope depends on electrostatic interactions

In biological applications of atomic force microscopy, the different surface properties of the biological sample and its support become apparent. Observed height differences between the biomolecule and its supporting surface are thus not only of structural origin, but also depend on the different sample-tip and support-tip interactions. This can result in negative or positive contributions to the measured height, effects that are described by the DLVO (Derjaguin, Landau, Verwey, Overbeek) theory. Experimental verification shows that the electrostatic interactions between tip and sample can strongly influence the result obtained. To overcome this problem, pH and electrolyte concentration of the buffer solution have to be adjusted to screen out electrostatic forces. Under these conditions, the tip comes into direct contact with the surface of support and biological system, even when low forces required to prevent sample deformation are applied. In this case, the measured height can be related to the thickness of the native biological structure. The observed height dependence of the macromolecules on electrolyte concentration makes it possible to estimate surface charge densities.

S-layer stabilized solid support lipid bilayers

This work describes composite structures composed of lipid bilayer or tetraetherlipid monolayer films attached to solid supports with associated crystalline bacterial cell surface layers (S-layers). The bilayer system was established by making use of the strong chemisorption of a first monolayer of thiolipids (1-octadecanethiol or 1,2-dimyristoyl-sn-glycero-3-phosphothioethanol) on gold and attaching a second monolayer of 1,2-dipalmitoyl-sn-3-phosphatidylethanolamine by the Langmuir Schaefer technique. The tetraetherlipid monolayer was composed of Glycerol-dialkyl-nonitol tetraetherlipid (GDNT). The monolayer of GDNT exhibits the thickness of a bilayer with hydrophilic headgroups on both sides and a hydrophobic inner part. Isolated S-layer protein from Bacillus sphaericus (CCM2177, which was injected into the subphase of an LB-trough, recrystallized into a coherent monolayer at the solid supported phospholipid bilayer and at the tetraehtherlipid monolayer. The composite lipid/S-layer structures were stable enough to allow lifting from the air-water interface, rinsing in water, and transfer into a scanning force microscope.

Chloroplast F0F1 ATP Synthase Imaged by Atomic Force Microscopy

The F0F1 ATP synthase of chloroplasts was imaged using atomic force microscopy (AFM) in contact mode under physiological conditions. Chloroplast (CF0F1) ATP synthases were reconstituted into liposomes. Liposomes were adsorbed on a mica surface where they spread and formed lipid bilayers containing CF0F1 ATP synthases which could be imaged. From these reconstituted CF0F1 ATP synthases, the CF1 part could be removed either by application of a chemical denaturant or less efficiently by mechanical stripping with the AFM tip. Embedded in the lipid bilayer were seen ring-like structures with a central dimple with outer diameters of 20 +/- 3 nm (chemical denaturant) and ca. 7 nm (mechanical stripping), respectively. Ring-like structures were also observed in a protein-free lipid bilayer. These had diameters of 30 +/- 5 nm and could be clearly distinguished from the structures observed after mechanical stripping. Hence, the ring-like structures observed after mechanical stripping might represent the intrinsic membrane domain CF0 or the oligomer of its subunit III. In addition, isolated CF1 adsorbed directly onto the mica surface was imaged. In accordance with the size known from electron microscopy, a diameter of 13 +/- 4 nm was measured.

Adsorption of biological molecules to a solid support for scanning probe microscopy

Scanning probe microscopes are now established tools to study the surface structure of biological macromolecules under physiological conditions. Sample preparation methods for this microscopy all have the objective to attach the specimen firmly to a support. Here we analyse the commonly used method of adsorbing biological specimens to freshly cleaved mica. This is facilitated by adjusting the electrolyte concentration and the pH of the buffer solution. Native macromolecular systems absorbed to mica in this way can be reproducibly imaged at submolecular resolution.